This invention relates to the machining of rotatable braking surfaces and, more particularly, it relates to machining of cast iron brake drums and disk brake rotors by ultrasonically machining the braking surface with an ultrasonic tool having a set of freely moving impacting elements driven by an oscillating ultrasonic transducer.
Braking surfaces, particularly those oriented on braking rotors, such as cast iron drums and disc brake rotors, have conventionally been tooled during primary manufacture by lathe cutting tools to present dimensioned machined surfaces having required tolerances. This manufacturing process produces in such braking rotors, in the form of brake discs or brake drums, a surface roughened with tooling marks and characterized by specific surface strength and hardness, a reformulated surface finish and tooling marks presenting a non-uniform stress profile and a source of tensile stress concentration. These tooling marks and tensile stresses limit the active service life of the braking surfaces thus encouraging fatigue cracks by heat checking and scuffing.
This problem has been addressed in U.S. Pat. No. 5,193,375, T. J. Meister, Mar. 16, 1993, entitled METHOD FOR ENHANCING THE WEAR PERFORMANCE AND LIFE CHARACTERISTICS OF A BRAKE DRUM and in U.S. Pat. No. 5,352,305, L. B. Hester, Oct. 4, 1994 and U.S. Pat. No. 5,664,648, L. B. Hester, Sep. 9, 1997, each entitled PRESTRESSED BRAKE DRUM OR ROTOR. These patents disclose a shot peening cold working method of treating the initially manufactured braking surface thereby inducing greater roughness in the braking surface. This peening creates a displacement of surface material about an impact point to establish a rim pattern of roughened peaks, to mask former tooling marks and tensile forces inherently introduced by the initial tooling procedure.
Objection can be taken to the roughened braking surface caused by peening. For example, such roughened braking surfaces require an intense burnishing process which develops heat checking when confronting the topography of brake pad surfaces thus encouraging heat checking hot spots which reduce the service life of the braking surface.
Furthermore, because of the nature of the product, particularly the confined and inaccessible interior braking surfaces of cast iron brake drums, it is physically difficult with peening methods to rework the braking surface areas without overlapping onto adjacent surfaces. Also precise and uniform control in the peening process to repeatedly meet different braking surface specifications is rarely feasible and practically impossible, thus leading to a low reliability of the peening method.
Proposed use of ultrasonic energy to restructure butt welded seams with an ultrasonic horn transducer directly in contact with the weld seam joining thin titanium sheets is taught by S. E. Jacke in U.S. Pat. No. 3,274,033, Sep. 20, 1966 for ULTRASONICS. However, the disclosed method and equipment would not be operable for processing the heavy cast iron structure used in brake drums and rotors, nor would it be able to process the sub-surface material to the depths needed for meeting braking surface specifications.
Furthermore, the ultrasonic treatment of welds requires a completely different relationship of ultrasonic impacts on the work body surface to strengthen welds. Rather, the processing of braking surfaces to achieve improvements of braking surface strength for the compressive interfacing between the braking surface and braking pads affects the work product in an entirely different way.
Prior ultrasonic impact treatment methods have been utilized for treating weld joints and fractures in structural iron, but it has not been known how to treat the topography and strength of braking surfaces ultrasonically to obtain greater strength and ability to perform the in-service role of compressively confronting brake pads.
Prior art brake drums are in general subject to thermo-mechanical fatigue, and propensity to develop cracks. The braking surfaces between drums and brake pads do not have appropriate shape and texture for service and thus need be subjected to extensive break-in procedures which themselves tend to produce cracking and thermo-induced stresses. The peening method of brake surface treatment leaves a roughened braking surface that reduces the amount of effective area of the braking surface in use, produces uneven distribution of individual peened peaks and contact area sizes to thus have the propensity to develop hot spots, thereby seriously limiting the maximum braking moment and contributing to early fatigue failures.
Further significant limitations of the peening method include the depth of allowable wear on the working surface and the uniformity of braking characteristics during the life of the brake drum.
One further limitation of the peening method is the inability to control the process to meet variable specifications. For example, the only control over braking surface roughness is the complicated matter of finding and implementing the appropriate shot sizes and accompanying striking forces. There is essentially no way to achieve a smooth cylindrical interface fit with an associated brake pad with the peening method.
Background technology for the application of ultrasonic impact energy to the surface of polypropylene and thermoplastic materials for welding or riveting, is evidenced by U.S. Pat. No. 5,976,314, Nov. 2, 1999 by Manfred Sans for DEVICE FOR ULTRASONIC TREATMENT OF WORKPIECES. However, this teaching does not disclose a feasible system for the reworking of machined metal work pieces by ultrasonic machining methods in a manner reworking the sub-surface crystalline structure and establishing the surface texture of a metal workpiece, as does this present invention.
Similarly, various specialty ultrasonic metal working impact transducers, including hand operated tools, are disclosed in the prior art for surface deformation and subsurface plasticization of explicit shapes and contours, typically cylinders, rotating surface segments and planar sheets by direct mechanical interfacing of an ultrasonically vibrating head with a metallic surface work site. Typical disclosures are found in Russian inventor""s certificates including: SU 1447646 A1, published Dec. 30, 1988; SU 1263510 A2, published Oct. 15, 1986; SU 1756125 A1, published Aug. 23, 1992; SU 1255405 A1, published Sep. 7, 1986; SU 1576283 A, published Jul. 7, 1990; SU 998104, published Jan. 5, 1981; SU 1214396 A, published Feb. 28, 1986; SU 1481044 A, published Sep. 28, 1987; and SU 1703417 Al, published Jan. 7, 1982 relating to direct mechanical contact between an oscillating ultrasonic transducer head oscillating at the prescribed ultrasonic frequency and the treated metallic object surface.
French 2,662,180 filed May 7, 1991 relates to a system for applying ultrasonically oscillating thermal impulse energy to induce plastic surface deformation at weld sites to improve the initial weld strength in the plastic material. This system does not disclose how to successfully apply ultrasonic energy to correct braking problems by machining brake drum surfaces.
Statnikov, et al published documents IIW XIII-1617-96 and IIW XIII-1609-95 relating to the state of the art of hand held tools for applying ultrasonic impact energy directly from an oscillating transducer head at the impacting resonant frequency of the driving oscillator. These transducers are special purpose transducers with a single impacting needle adapted to a system configuration for achieving the particular functional treatment of strengthening welded structure of a specific workpiece configuration at a specific periodic ultrasonic resonant frequency.
Thus, this state of the ultrasonic transducer prior art will permit one skilled in the art to select and apply appropriate ultrasonic transducers for practicing this invention.
It is a general objective of the present invention to provide improved methods of machining metallic braking surfaces, typically cast iron by way of ultrasonic machining of the braking surfaces to restructure the surface strength and topography in a manner significantly increasing braking service strength and life.
It is a further object of the invention to overcome by ultrasonic machining methods the disadvantages of resident tooling marks on the braking surface, which produce tensile stress concentrations.
In general it is an object of this invention to process braking surfaces, typically cast iron brake drums, as a substantially final manufacturing step.
Another object of the invention is to machine the cylindrical interior drum braking surfaces with ultrasonic transducer assemblies which are small enough to be positioned inside the drum cylinder to establish precise working dimensions over braking surfaces that better mate with the drum confronting surfaces of the brake pads and which are confined to the braking surfaces only.
A more specific object of the invention is to plastically deform a braking surface by ultrasonic machining methods to increase contact area and strength of the braking surface metal and adjacent sub-surface region to a typical depth exceeding 500 microns and up to 3000 microns for meeting the designated wear depth tolerances of particular braking system surfaces.
Another objective of the invention is to develop improved smoothed braking surface topography on braking surfaces modified to remove roughened tooling marks which better conform with mating brake pads typically with macro-relief of less than 200 micro inch and a smoother finish micro-relief, thus advantageously providing a better surface for interfacing with the brake lining, better dissipation of heat build up, and increased surface contact for braking purposes.
It is another object of the invention to produce novel cast iron brake drums with ultrasonically processed modifications of the braking surface in the final manufacturing procedure therefore providing uniform distribution of residual deformations up to about 12 mm in depth, which deformations retain their distribution during life and afford greater braking strength.
Other objectives will be found throughout the following description, drawings, and claims.
The above identified parent applications are incorporated herein in their entirety by reference.
The above objectives are achieved by employing novel ultrasonic machining methods as a final stage machining process in the manufacture of brake drums, in particular for removing former machine tooling marks. This results in novel braking surface re-manufacturing methods particularly related to cast iron surface structure of brake drums and brake rotors which produce designated smoothness of the braking surface configurations presenting greater brake surface contact area than obtainable by peening methods and thus producing much higher braking strengths.
To achieve these objectives, this invention introduces novel ultrasonic machining systems that uniquely machine braking surface structure to produce the desirable smooth rotor braking surfaces and strengthened braking surfaces which impart longer life braking service with increased braking forces.
Smooth braking surfaces are produced to facilitate mechanical abutment and engagement of the braking surface and its mating brake pads and thus provide more braking surface contact than the prior art peening-roughened surfaces.
These novel ultrasonic impact machining methods are interrelated and dependent upon ultrasonic tooling systems capable of working directly in the restricted access region of the drum brake internal cylindrical braking surface to precisely attain and repeatedly meet brake surface specifications over the entire braking surface in an efficient and low cost manner.
Thus, an ultrasonic transducer head adapted for efficient transfer of oscillating ultrasonic energy at frequencies up to 55 kHz from an ultrasonic periodically vibrating energy transfer surface is employed. A set of freely axially movable impacting elements is interposed between a transducer energy transfer surface and the braking surface being machined in a braking surface scanning mode to uniformly cover the entire braking surface.
A scanning system employing a lathe unit positions the transducer and impact members inside the cylindrical brake drum for ultrasonically impacting only the exposed braking surface without machining any other exposed surfaces as is required by the nature of the prior art peening methods. Thus the set of transducer driven indenter members scan the braking surface as the brake drum and transducer impact members are relatively rotated therewith to establish a substantially uniformly machined braking surface.
This novel mode of operation is thus advantageous over the former peening methods which by nature impact areas outside of the braking surface thereby dissipating available kinetic energy per unit of braking surface area. By producing smoother controlled braking surface macro and micro surface structure provides a much larger braking impact area to achieve plastic deformation and result in a higher braking strength of the cast iron braking surface. The prior art peening mode of operation serves to indent and reshape the impact area into the peaks and valleys of roughened surface texture which eliminates braking surface contact over a substantial portion of the braking surface.
The ultrasonic impact machining methods of this invention with associated ultrasonic systems features therefore produce a superior higher strength machining of the braking drum or rotor to present smoothed surfaces to accommodate specified braking system requirements and to provide higher braking forces in service with a larger mating surface interfacing with a brake pad in dry sliding contact with increased braking forces to effect frictional loading over a longer life of braking service.
The braking surface is plastically modified by the ultrasonic methods applied with the ultrasonic transducer to achieve a two-component surface texture with macro and micro components, and to strengthen the surface material, typically cast iron, beyond the yield strength and approaching the ultimate strength of the base material, thereby creating internally in adjacent sub-surface regions a normalized deflection mode of the material in the brake drum body realized by balancing of internal forces. Employment of an associated set of simultaneously oscillating indenters in the transducer head produces the novel characteristics of a smoother more uniform interfacing surface machined on the drum during a scanning operation.
Other features and advantages of the invention will be found throughout the following drawings, description and claims.